The expansion of Phytophthora clade 8b: three new species associated with winter grown vegetable crops

Despite its association with important agricultural crops, Phytophthora clade 8b is a poorly studied group of species. The clade currently consists of three officially described species (Phytophthora porri, P. brassicae and P. primulae) that are host-specific pathogens of leek, cabbages and Primula spp., respectively. However, over the past few decades, several other clade 8b-like Phytophthoras have been found on a variety of different host plants that were all grown at low temperatures in winter seasons. In this study, a collection of 30 of these isolates was subjected to a phylogenetic study using two loci (the rDNA ITS region and the mitochondrial cox1 gene). This analysis revealed a clear clustering of isolates according to their host plants. To verify whether these isolates belong to separate species, a detailed morphological study was conducted. On the basis of genetic and morphological differences and host specificity, we now present the official description of three new species in clade 8b: Phytophthora cichorii sp. nov., P. dauci sp. nov. and P. lactucae sp. nov. Two other groups of isolates (Phytophthora taxon castitis and Phytophthora taxon parsley) might also represent new species but the data available at this time are insufficient for an official description. This brings Phytophthora clade 8b to a group of six species that are all host-specific, slow-growing and specifically infect herbaceous crops at low temperatures.